The present invention generally relates to quantum semiconductor devices and more particularly to a quantum semiconductor memory device that uses a quantum dot structure for storage of information and a fabrication process thereof.
In a so-called bulk semiconductor crystal in which there is no carrier confinement, the state density of carriers increases continuously along a parabolic curve with energy. In a so-called quantum well structure in which the carriers are confined one-dimensionally in a two-dimensional plane, on the other hand, there appears a stepwise change in the state density with energy due to the existence of quantum states. In such a system that has a stepwise change in the state density, the distribution of carriers is restricted substantially as compared with the case of a bulk crystal, and a sharp optical spectrum is obtained when the quantum well structure is used for an optical semiconductor device such as a laser diode. By using a quantum well structure, the efficiency of optical emission is improved in such optical semiconductor devices. Further, quantum well structures are used for an energy filter of carriers in electron devices that have a resonant-tunneling barrier such as an RHET (resonant-tunneling heterostructure transistor).
In a quantum wire structure in which the degree of carrier confinement is increased further, the state density of the carriers is modified such that there appears a maximum state density at the bottom edge of each step. Thereby, the sharpness of the energy spectrum increases further.
In an ultimate quantum dot structure in which the degree of carrier confinement is increased further, the state density becomes discrete as a result of the three-dimensional carrier confinement, and the energy spectrum of the carriers becomes also discrete in correspondence to the discrete quantum levels. In the system that has such a discrete energy spectrum, the transition of carriers occurs discontinuously between the quantum levels even at a room temperature in which there exists a substantial thermal excitation. Thus, an optical semiconductor device that uses a quantum dot structure can provide a very sharp optical spectrum even in a room temperature operation.
Further, the energy filter having a quantum dot structure can provide the desired very sharp energy spectrum not only at a very low temperature but also at a room temperature.
Meanwhile, there is a proposal to construct a quantum semiconductor memory device that uses such a quantum dot structure for an optical storage of information. For example, Muto, et al. (Muto, S. Jpn. J. Appl. Phys. vol.34, 1995, pp.L210-212, Part2, No.2B, February 1995) describes a quantum semiconductor memory device that uses a quantum dot structure formed on a stepped semiconductor surface by a lateral epitaxial growth of semiconductor layers. In the proposed structure, the electrons excited as a result of an optical excitation are caused to tunnel to an adjacent semiconductor layer and held therein.
In the foregoing quantum structure, the electron excited in the quantum dot is held stably in the semiconductor layer adjacent to the quantum dot in a spatially separated state from the hole that is created as a result of the optical excitation of the electron and remaining in the quantum dot. By forming the quantum dot from a semiconductor material of a direct-transition type and by forming the adjacent semiconductor layer from a semiconductor material of an indirect-transition type, the optical excitation of carriers in the adjacent semiconductor layer is effectively avoided.
FIG. 1 is a band diagram showing the principle of the conventional quantum semiconductor memory device of the foregoing prior art.
Referring to FIG. 1, the quantum semiconductor memory device includes a quantum dot M1 of GaAs surrounded by a storage layer M3 of AlAs, with a thin barrier layer M2 of AlGaAs intervening between the quantum dot M1 and the storage layer M3. It should be noted that GaAs forming the quantum dot M1 is a typical direct-transition type semiconductor material and causes an excitation of an electron represented by a solid circle to a quantum level L.sub.e and a hole represented by an open circle to a quantum level L.sub.h in response to an irradiation of an optical radiation having a wavelength .nu..
Thereby, it should be noted that the quantum dot M1 has a size set such that the quantum level L.sub.e is located higher than a bottom edge of the conduction band of the adjacent AlAs layer M3, so that the electron thus excited in the quantum dot M1 can fall to the conduction band of the AlAs layer M3, after passing through the barrier layer M2 by tunneling. On the other hand, the hole that is created in the quantum dot M1 as a result of the optical excitation of the electron remains in the quantum dot M1 because of the larger effective mass. Thereby, the electron and hole thus excited optically are held stably at respective locations separated from each other spatially.
In the band structure of FIG. 1, it should be noted that the optical excitation in the AlAs layer M3 does not occur substantially. In the AlAs layer M3, which is an indirect-transition type semiconductor, the optical excitation of carriers from the valence band to the X-valley of the conduction band occurs only in the presence of the other elementary excitation such as a phonon that satisfies the conservation of momentum. Further, the excitation to the .GAMMA.-valley, which does not require such an interaction with other elementary excitations, does not occur because of the very large transition energy necessary for causing the optical excitation. Thus, there occurs no substantial optical excitation in the AlAs layer M3.
In order to fabricate the quantum semiconductor memory device having such a quantum dot structure, it is necessary to establish a technology to form a high-quality quantum dot with clearly defined quantum levels so that the desired optical transition of carriers occurs between these quantum levels. In addition, it is necessary that the size of the quantum dot is controlled in the quantum semiconductor memory device of FIG. 1 such that the quantum level L.sub.e in the quantum dot M1 is located at an energetically higher level than the X-valley of the conduction band of the AlAs layer M3.
Conventionally, the so-called quantum well structure that confines the carries in a substantially two-dimensional surface has been formed successfully and with reliability by using an MBE (molecular beam epitaxy) process or an MOVPE (metal-organic vapor phase epitaxy) process, such that a very thin quantum layer is sandwiched by a pair of barrier layers. Further, a quantum wire structure, in which the carriers are confined substantially along a one-dimensional wire, can be formed by using a so-called inclined semiconductor substrate having a stepped structure on a principal surface thereof. It is known that a quantum wire can be formed by causing a lateral epitaxial growth of a narrow quantum semiconductor layer having a small thickness and a limited width from each lateral edge of the stepped structure along the stepped surface. Alternatively, a quantum wire may be formed by applying an electron beam lithography.
Thus, it has been thought that a quantum dot structure may also be formed by using a stepped surface of an inclined semiconductor substrate or kink similarly to the case of forming a quantum wire. It turned out, however, that it is difficult to control the stepped surface of the inclined semiconductor substrate such that the desired formation of the isolated quantum dot is formed according to such a lateral epitaxial process. Further, the tendency that a mixing of element occurs at the heteroepitaxial interface of the quantum dot thus formed according to such a lateral epitaxial process makes it difficult to form a clearly defined quantum dot having a boundary where there is a sharp change of composition.
Further, the process of forming a quantum dot by using a photolithographic process is not successful due to the substantial damages caused in the quantum dot as a result of the patterning process.
Because of the reasons noted above, the foregoing prior art quantum semiconductor device is not realized yet.
On the other hand, there has been a discovery that a quantum dot can be formed easily by using a so-called S-K (Stranski-Krastanow) growth mode that occurs in a strained heteroepitaxial system such as an InAs/GaAs heteroepitaxial structure at the initial period of heteroepitaxial growth. In an S-K growth mode, quantum dots are formed in the form of discrete islands on a substrate. For example, it is reported that an MBE growth of an InGaAs layer having an In-content of 0.5 on a GaAs substrate with a thickness of several molecular layers, results in a formation of islands of InGaAs each having a diameter of 30-40 nm on the GaAs substrate (Leonard, D., et al., Appl. Phys. Lett. 63, pp.3203-3205, 1993). Further, it is reported that islands of InGaAs having a diameter of 15-20 nm are formed on a GaAs substrate by an ALE (Atomic Layer Epitaxy) process with a mutual distance of about 100 nm (Mukai, K., et al., Jpn. J. Appl. Phys., 33, pp.L1710-L1712, 1994). Further, a similar quantum dot can be formed also by a MOVPE process (Oshinowo, J. et al., Appl. Phys. Lett. 65,(11), pp.1421-1423, 1994).
FIGS. 2A and 2B show an example of such a self-organized quantum dot formed in a strained heteroepitaxial system respectively in a cross sectional view and in a plan view.
Referring to FIG. 2A, a GaAs substrate 1 carries thereon a buffer layer 2 of AlGaAs, and another GaAs layer 3 is formed on the buffer layer 2 epitaxially. The GaAs layer 3 in turn carries thereon a quantum well layer 4 of InGaAs, wherein it should be noted that InGaAs forming the quantum well layer 4 has a lattice constant substantially larger than a lattice constant of GaAs forming the underlying layer 3. Thus, the quantum well layer 4 causes an island growth on the layer 3 as indicated in FIG. 2B, and a number of mutually isolated self-organized quantum dots of InGaAs, each having a height of several nanometers and a diameter of several tens of nanometers, are formed on the GaAs layer 3 spontaneously. By depositing a GaAs barrier layer 5 having a bandgap larger than the bandgap of InGaAs on the InGaAs quantum dot thus formed, discrete quantum levels are formed in the self-organized quantum dot.
As the formation of a self-organized quantum dot in such a strained heteroepitaxial system is controlled by a strain energy formed at the heteroepitaxial interface, the formation of the quantum dot is substantially simplified as compared with the conventional process discussed previously. Further, the formation of a quantum dot on a strained heteroepitaxial system does not require a patterning process and is inherently free from damages. There is already a report claiming successful observation of a photoluminescence (PL) (Leonard, D., et al., op. cit.), in which it is reported that a broad PL peak is confirmed in the vicinity of 1.2 eV with a substantial intensity.
In the quantum dots formed by the S-K growth mode, however, the observed PL peak, although having a substantial intensity, spreads or diffuses substantially. For example, the half-height width FWHM (Full Width at Half Maximum) of the PL peak spreads over a range of 80-100 meV, probably due to the poor control of the size of the individual quantum dots. Recently, Farad et al., (Farad. S., Appl. Phys. Lett., 68(7), pp.991-993, Feb. 12, 1996) has reported a successful observation of a PL wavelength in the 1.5 .mu.m-band for an S-K mode quantum dot of InAs formed on an AlInAs buffer layer, which in turn is provided on an InP substrate with a lattice matching therewith. In this case, however, the value of FWHM for the observed PL spectrum exceeds 110 meV, indicating that there still remains a substantial problem in the size control of the individual quantum dots.
On the other hand, the existence of such a variation in the size of the quantum dots and hence the wavelength of the optical radiation interacting with the quantum dots, is advantageous for constructing a quantum semiconductor device. In such a quantum semiconductor memory device, it is possible to write different information into the same region of a recording medium, in which the quantum dots are formed, by changing the wavelength of the optical beam used for recording the information. In other words, an optical semiconductor memory device that records information in a wavelength multiplex mode is obtained easily and simply, by using the self-organized quantum dots formed by the S-K growth mode.
In such an optical semiconductor memory device that uses the self-organized quantum dot structure formed by the S-K growth mode, the size of the individual quantum dot is not controlled by the designer of the device as noted already. The size of the quantum dot is roughly determined by the materials used for the substrate and the quantum dot or the deposition temperature. Thus, it has not been possible to control the quantum level L.sub.e of the quantum dot M1 such that the quantum level L.sub.e is located above the X-valley of the conduction band of the adjacent AlAs layer M3, contrary to the case of FIG. 1. Thus, it has not been obvious at all that the desired writing, holding or reading of information is possible when the self-organized quantum dot is applied directly to the quantum semiconductor memory device of FIG. 1.